Re: [PATCH] mm/slub: don't wait for high-order page allocation

[Joonsoo Kim <iamjoonsoo.kim@lge.com>]

On Tue, Aug 04, 2015 at 03:15:26PM +0200, Michal Hocko wrote:
> On Fri 31-07-15 10:09:50, Joonsoo Kim wrote:
> > Almost description is copied from commit fb05e7a89f50
> > ("net: don't wait for order-3 page allocation").
> > 
> > I saw excessive direct memory reclaim/compaction triggered by slub.
> > This causes performance issues and add latency. Slub uses high-order
> > allocation to reduce internal fragmentation and management overhead. But,
> > direct memory reclaim/compaction has high overhead and the benefit of
> > high-order allocation can't compensate the overhead of both work.
> > 
> > This patch makes auxiliary high-order allocation atomic. If there is
> > no memory pressure and memory isn't fragmented, the alloction will still
> > success, so we don't sacrifice high-order allocation's benefit here.
> 
> But you are also giving those allocations access to a portion of the
> memory reserves which doesn't sound like an intenteded behavior here.
> At least the changelog doesn't imply anything like that.
> 
> I am not oppposed to your patch but I think we should do something about
> the !__GFP_WAIT behavior. This is too subtle and the mere fact the
> caller doesn't want or cannot sleep doesn't make it a reserve consumer
> automatically. We have __GFP_HIGH for that purpose. If this is not
> desirable because of the regression risk then we might need a new gfp
> flag for a best effort allocation which will fail in case we have to
> dive into costly reclaim.

Hello, Michal.

We already have __GFP_NOMEMALLOC not to use emergency pool in case of
!__GFP_WAIT. Please see gfp_to_alloc_flags().
I'll send update version to use this flag.

BTW, network commit fb05e7a89f50 doesn't specify this flag. Does it
need this change, too?

Thanks.

> 
> > If the atomic allocation fails, direct memory reclaim/compaction will not
> > be triggered, allocation fallback to low-order immediately, hence
> > the direct memory reclaim/compaction overhead is avoided. In the
> > allocation failure case, kswapd is waken up and trying to make high-order
> > freepages, so allocation could success next time.
> > 
> > Following is the test to measure effect of this patch.
> > 
> > System: QEMU, CPU 8, 512 MB
> > Mem: 25% memory is allocated at random position to make fragmentation.
> >  Memory-hogger occupies 150 MB memory.
> > Workload: hackbench -g 20 -l 1000
> > 
> > Average result by 10 runs (Base va Patched)
> > 
> > elapsed_time(s): 4.3468 vs 2.9838
> > compact_stall: 461.7 vs 73.6
> > pgmigrate_success: 28315.9 vs 7256.1
> > 
> > Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>
> > ---
> >  mm/slub.c | 2 ++
> >  1 file changed, 2 insertions(+)
> > 
> > diff --git a/mm/slub.c b/mm/slub.c
> > index 257283f..2d02a36 100644
> > --- a/mm/slub.c
> > +++ b/mm/slub.c
> > @@ -1364,6 +1364,8 @@ static struct page *allocate_slab(struct kmem_cache *s, gfp_t flags, int node)
> >  	 * so we fall-back to the minimum order allocation.
> >  	 */
> >  	alloc_gfp = (flags | __GFP_NOWARN | __GFP_NORETRY) & ~__GFP_NOFAIL;
> > +	if ((alloc_gfp & __GFP_WAIT) && oo_order(oo) > oo_order(s->min))
> > +		alloc_gfp = alloc_gfp & ~__GFP_WAIT;
> >  
> >  	page = alloc_slab_page(s, alloc_gfp, node, oo);
> >  	if (unlikely(!page)) {
> > -- 
> > 1.9.1
> > 
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> 
> -- 
> Michal Hocko
> SUSE Labs
> 
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